6'-SL may increase the brain bioavailability of its active form, sialic acid, via slow hydrolysis and absorption as well as reduced excretion, while 3'-SL exhibits higher intestinal absorption efficiency in an energy-dependent manner.
Key Findings
Results
Sialyllactoses are hydrolyzed into free sialic acid (SA) in the large intestine after oral administration.
Both 3'-SL and 6'-SL underwent hydrolysis to release free sialic acid as the primary metabolite.
SA was identified as the primary metabolite of SLs during in vitro fermentation.
Hydrolysis occurred predominantly in the large intestine rather than earlier in the gastrointestinal tract.
The study used an ADME (absorption, distribution, metabolism, and excretion) framework to track SL fate in vivo.
Results
The concentration of free sialic acid in the intestine was higher in the 6'-SL group than in the 3'-SL group.
After oral administration, intestinal SA concentrations differed between the two isomers.
Higher intestinal SA in the 6'-SL group is consistent with slower hydrolysis and reduced absorption from the intestinal lumen.
This finding suggests 6'-SL releases SA more gradually in the gut compared to 3'-SL.
The differential intestinal SA levels reflect the distinct metabolic profiles of the two sialyllactose isomers.
Results
Conjugated sialic acid levels in the brain were higher in the 6'-SL group than in the 3'-SL group.
Brain conjugated SA was measured as an indicator of bioavailability of the active form of SA in the central nervous system.
Higher brain conjugated SA in the 6'-SL group suggests greater delivery of SA to the brain following 6'-SL administration.
This finding supports the hypothesis that 6'-SL increases brain bioavailability of SA via slow hydrolysis and reduced excretion.
The difference in brain SA levels between 3'-SL and 6'-SL highlights functionally relevant pharmacokinetic distinctions between the two isomers.
Results
Conjugated sialic acid levels in serum and urine were higher in the 3'-SL group than in the 6'-SL group.
Serum and urinary conjugated SA were both elevated in the 3'-SL group relative to the 6'-SL group.
Higher urinary SA excretion in the 3'-SL group indicates greater systemic clearance of SA derived from 3'-SL.
Reduced excretion in the 6'-SL group is proposed as a mechanism contributing to greater brain SA availability.
These findings suggest that SA absorbed from 3'-SL is more readily excreted rather than retained in target tissues such as the brain.
Results
3'-SL was absorbed in Caco-2 cells in an energy-dependent manner, with higher absorption efficiency than 6'-SL.
Caco-2 cell monolayers were used as an in vitro model of intestinal absorption.
The absorption of 3'-SL was characterized as energy-dependent, indicating involvement of active transport mechanisms.
Absorption efficiency of 3'-SL was higher than that of 6'-SL in the Caco-2 model.
The energy-dependent nature of 3'-SL absorption distinguishes its transport mechanism from that of 6'-SL.
Higher absorption efficiency of 3'-SL in the intestine is consistent with its higher serum SA levels observed in vivo.
Results
The hydrolysis rate of 6'-SL was lower than that of 3'-SL during in vitro fermentation.
In vitro fermentation experiments were used to assess microbial metabolism of SLs.
SA was confirmed as the primary metabolite of both SLs during fermentation.
The slower hydrolysis rate of 6'-SL compared to 3'-SL was observed under fermentation conditions mimicking the gut microbiome.
Slower hydrolysis of 6'-SL is proposed as one mechanism explaining the higher availability of SA in the intestine and brain of the 6'-SL group.
The differential hydrolysis rates reflect structural differences between the α2,3 and α2,6 glycosidic linkages of 3'-SL and 6'-SL, respectively.
Discussion
The study identifies a dynamic interplay between intestinal absorption and microbial metabolism as the main driver of bioavailability differences between 3'-SL and 6'-SL.
Both the rate of microbial hydrolysis and the efficiency of intestinal absorption contribute to the differential distribution of SA derived from the two SL isomers.
6'-SL's slower hydrolysis and lower intestinal absorption efficiency, combined with reduced urinary excretion, together increase brain SA bioavailability.
3'-SL's higher absorption efficiency leads to greater systemic exposure but also greater urinary excretion of SA.
The authors conclude that 'the bioavailability of 6'-SL may increase the brain bioavailability of its active form, SA, via slow hydrolysis and absorption as well as reduced excretion.'
What This Means
This research investigated how two closely related compounds found in human breast milk — called 3'-sialyllactose (3'-SL) and 6'-sialyllactose (6'-SL) — are processed differently by the body after being consumed. Both compounds belong to a class of sugars known as human milk oligosaccharides, and both are broken down in the large intestine into a simpler molecule called sialic acid (SA), which is thought to be the biologically active component. The study used animal experiments, laboratory cell models, and test-tube fermentation experiments to track how each compound is absorbed, distributed, broken down, and excreted.
The researchers found important differences between the two compounds. 3'-SL was absorbed more efficiently from the intestine into the bloodstream, but this also meant more of its breakdown product (sialic acid) ended up being excreted in the urine rather than reaching the brain. In contrast, 6'-SL was broken down more slowly by gut bacteria and absorbed less efficiently from the intestine, which left more sialic acid available in the intestinal environment. Strikingly, despite lower blood levels, the 6'-SL group showed higher levels of sialic acid in the brain. This suggests that 6'-SL's slower processing is actually advantageous for delivering sialic acid to the brain.
This research suggests that 6'-SL, through its slower hydrolysis by gut microbes and lower intestinal absorption rate combined with reduced urinary excretion, may be more effective at increasing brain levels of sialic acid compared to 3'-SL. Since sialic acid is important for brain development and function, these findings could have implications for the formulation of infant formula and nutritional supplements that include sialyllactose ingredients. The study highlights that structural differences between very similar molecules can lead to meaningfully different biological outcomes in the body.
Li X, Zhang Y, Xu F, Ji X, Wang J, Pan C, et al.. (2026). Bioavailability of 6'-Sialyllactose and 3'-Sialyllactose: Dynamic Interplay between Intestinal Absorption and Microbial Metabolism as a Main Driver.. Journal of agricultural and food chemistry. https://doi.org/10.1021/acs.jafc.5c13148